1. Field of the Invention
The present invention relates to the field of power transmission systems for helicopters and other devices and, more particularly, to a novel large gear reduction power transmission including an extremely lightweight, simple and efficient transmission having a minimum number of gear meshes in series, which, when used in conjuction with heavy lift helicopters, can extend the gross lifting weight and payload of this type of aircraft for beyond that which is now possible.
2. Definition of Problem
In the design and construction of any airborne vehicle, weight reduction is always of critical concern and is directly related to the payload available for transport in the vehicle. In such vehicles as heavy lift helicopters, the transmission operably connected between the engine and the lifting rotor presents an additional weight problem. Helicopter transmission weight increases along an exponential curve as the rotor diameter is increased because rotor tip speed, disk loading and power loading are not functions of helicopter gross weight and size. Rotor tip speed is usually about 700 feet per second and disk loading (gross weight per square foot of rotor area) and power loading (gross weight per horse power) usually fall within the curves shown in FIG. 3.
The result of the above facts is that gross weight and power go up by the square of the rotor diameter and torque goes up by the cube because RPM (for the same rotor tip speed) is inversely proportional to RPM. Transmission weight obviously goes up with an increase in torque and overall gear ratio.
In conventional size helicopters this phenomenon does not make too significant an impact, but as gross weight increases in heavy lift helicopters, this exponential relationship between transmission weight and gross weight can be extrapolated to the point where payload is completely taken over by transmission weight. Obviously, the cost effective size of a helicopter falls far below this point.
FIG. 4 graphically depicts the phenomenon discussed above. If line 17 represents the sum total of payload, fuel and transmission weight, and line 18 represents the weight of a conventional helicopter transmission, and line 19 represents the weight of the transmission using the present invention, it can be seen that the vector length (shown between the arrowheads) of lines 21, 22, 23 and 24 represent payload and fuel at the gross weights shown for both vehicles. With a conventional transmission, the maximum possible payload is line 21 where its intersection with line 18 is at the tangent point of 18 and 25 which is parallel to line 17. When gross weight reaches the intersection of lines 17 and 18, the payload and fuel is entirely used up by transmission weight. The practical gross weight is at a point way down the curve on line 22 which is dictated by cost effectiveness and actual payload requirements.
Line 19 has the same general shape as line 18 but it falls much further out on the gross weight curve which in turn produces a much greater payload. It can be seen that line 24 represents the maximum payload possible and line 23 represents a cost effective payload comparable to payload 22 using the conventional transmission. The difference in these payloads is substantial.
3. Approach to Problem Solution
The weight of a gear mesh in any transmission is a function not only of tooth load but also of the number of teeth which are loaded at any one instant of time in relation to the total number of teeth in the mesh. As tooth load is indirectly proportional to the number of input pinions in a divided load path gear mesh, and as the percentage of loaded teeth to total teeth goes up with an increase of input pinions, it is therefore obvious that weight can be decreased substantially by splitting the power in several parallel paths by increasing the number of input pinions driving a common putput gear and by increasing the overall transmission efficiency by reducing the number of gear stages in series by having a large gear reduction at the final stage.
4. Brief Description of Prior Art
Existing helicopters usually use planetary type gear meshes in their rotor transmissions, especially in the final stage where weight is the highest due to low RPM and high torque requirements. Planetary gears can transmit more power for less weight than a conventional pinion and gear because it divides the torque into several planetary gears thus allowing lower tooth loading of each gear mesh.
Unfortunately, due to the geometric constraints of a planetary gear mesh, the number of planetary gears used to divide the tooth loads is limited by the gear ratio of the mesh. If a large overall gear ratio is required, more meshes are needed in series to provide the total. For example, a four to one gear ratio in a simple planetary mesh cannot have more than five planetary gears. It is therefore impossible to obtain a high gear ratio with a standard planetary mesh and have a high number of planetary gears at the same time.
Much of the fundamental reason for splitting the torque in planetary meshes is lost when two or more planetary meshes are combined in series. A single input of one mesh produces a single output which becomes the single input of the next stage. Multi engined helicopters which start with as many load paths as they have engines must combine their separate power from each engine into a single input gear of the planetary mesh from adjoining gear boxes requiring support bearings and heavy structure to maintain rigidity between the pitch diameters of the mating teeth.
Load factors placed on planetary meshes also add weight. Although the sun gear of a planetary mesh usually floats at the center of the planet carrier gear cluster, when more than three planet carrier gears are used, a load factor must be added to all the gears in the mesh to allow for uneven load distribution due to deflections and manufacturing tolerances.
Using a planetary mesh as the final stage of a transmission is therefore not the lightest method of converting power from high RPM at low torque to low RPM at high torque.
A long standing need has existed to provide a lighter and more efficient heavy lift helicopter transmission.
State of the art improvements in metallurgy, detail design techniques, etc. have reduced transmission weight fractions somewhat. These improvements, however, are on the "flat part of the curve". Future weight savings will be small in these areas.
In order to substantially extend the gross weight and payload of heavy lift helicopters, an entirely new design concept is required to make the quantum jump in transmission weight reduction in order to meet and satisfy the heavy payload requirements of the trade.